1. Field of the Invention
The present invention generally relates to a hysteresis detecting method and a hysteresis detecting circuit applied to various kinds of electronic devices, and particularly to a hysteresis detecting method and a hysteresis detecting circuit suitable for detecting the intensity of illumination of mobile devices and to a mobile device including this hysteresis detecting circuit.
2. Description of the Prior Art
Recently, since various kinds of mobile devices, such as cell phones, are becoming smaller in size and lighter in weight and are driven by a battery power supply, it has been requested that they should be driven during a long duration with a small power consumption. A mobile device has a display unit (for example, a KEY display unit of a cell phone, etc.). This KEY display unit is energized by a backlight, customarily a light-emitting diode (LED), and becomes brighter. When the backlight of these mobile devices is energized, the backlight is kept brighter than the ambient illuminance necessitates, and it becomes a serious factor which deteriorates power consumption of the mobile device. For this reason, the mobile device uses an illuminance sensor to detect ambient illuminance and controls the KEY display unit to have the necessary intensity of illumination relative to ambient illuminance, thereby decreasing power consumption.
A circuit for detecting ambient illuminance to control a KEY display unit will be described below.
A circuit for controlling a KEY display unit is based upon a method in which an illuminance voltage, which results from converting a current flowing through an illuminance sensor in response to ambient illuminance by a resistor, and a reference voltage, set to the intensity of illumination to be detected, are compared with each other and in which a signal for energizing or de-energizing the KEY display unit based on a relationship between the magnitudes of the two voltages is used. That is, when the illuminance voltage is higher than the reference voltage, the ambient illuminance is higher than the intensity of illumination to be detected and hence the KEY display unit is de-energized. When the illuminance voltage is lower than the reference voltage, the ambient illuminance is lower than the intensity of illumination to be detected and hence the KEY display unit is energized. However, when the intensity of illumination is detected in actual practice, in order to prevent the KEY display unit from blinking in the state in which ambient illuminance near the intensity of illumination to be detected (that is, illuminance corresponding to a voltage close to the reference voltage) is inputted, a reference voltage is required to have hysteresis.
FIG. 1 of the accompanying drawings is a schematic diagram showing an example of a detecting circuit having hysteresis (hereinafter referred to as a “hysteresis detecting circuit”) according to the prior art. As shown in FIG. 1, this hysteresis detecting circuit 41 is of the low power consumption type and comprises a differential amplifier 40, a switch 48 and a comparator (hereinafter referred to as a “comparator circuit”) 43 composed of voltage supplying means 44, 45 for selectively switching the switch 48 to supply two reference voltages Vref1, Vref2 to the reference input terminal (positive input terminal) of the differential amplifier 40. An input voltage Vin to be compared is inputted to an input terminal (corresponding to the negative input terminal of the differential amplifier 40) of the comparator circuit 43. In FIG. 1, reference numeral 42 denotes a power supply.
Operations of this hysteresis detecting circuit 41 will be described next. As shown in FIG. 1, in the hysteresis detecting circuit 41, an input voltage Vin to be compared is inputted to the negative input terminal 46 of the comparator circuit 43. When the compared input voltage Vin becomes higher than the voltage values of the reference voltages Vref1, Vref2, the output voltage Vout outputted from the output terminal 47 of the comparator circuit 43 goes to a low level (Low). Conversely, when the compared input voltage Vin becomes lower than the voltage values of the reference voltages Vref1, Vref2, the output voltage Vout of the comparator circuit 43 goes to a high level (High). Also, the switch 48 inputs the low reference voltage Vref1 to the positive input terminal of the comparator circuit 43 when the output voltage Vout of low level (Low) is outputted from the output terminal 47, and it inputs the high reference voltage Vref1 to the positive input terminal of the comparator circuit 43 when the output voltage Vout of high level (High) is outputted from the output terminal 47.
Since the hysteresis detecting circuit 41 is formed of the low power consumption circuit arrangement, the reference voltages Vref1, Vref2 are slow to rise when the hysteresis detecting circuit 41 is actuated, and hence these reference voltages Vref1, Vref2 are stabilized slowly. As a result, when the hysteresis detecting circuit 41 is actuated, the compared input voltage Vin is stabilized earlier than the reference voltages Vref1, Vref2 and inputted to the comparator circuit 43. Thus, the output from the comparator circuit 43 is determined based on a relationship between the magnitudes of the reference voltages Vref1, Vref2 in the transient state and the input voltage Vin that has already been stabilized, whereby the reference voltages Vref1, Vref2 obtained when the hysteresis detecting circuit 41 is actuated are determined.
FIG. 2 shows the initial state attained when the compared input voltage Vin is inputted to the hysteresis detecting circuit 41 (shown in FIG. 1) in the state in which its level is lower than the levels of the reference voltages Vref1, Vref2. The initial state shows a period T1 from the detection start time, that is, the time at which the detection trigger pulse is outputted to the time at which the reference voltages Vref1, Vref2 are stabilized. When the aforementioned voltage is inputted to the hysteresis detecting circuit 41, since the compared input voltage Vin is higher than the reference voltages Vref1, Vref2 during the time period in which the hysteresis detecting circuit 41 is actuated to the point A, the output voltage Vout goes to the low level (Low) and the selector switch 48 inputs the low reference voltage Vref1 to the comparator circuit 43. However, after the point A, the relationship between the magnitudes of the voltages is reversed so that the output voltage Vout goes to the high level (High), and the selector switch 48 is allowed to input the high reference voltage Vref2 to the comparator circuit 43. That is, during a period Ta ahead of the point A, the selector switch 48 selects the reference voltage Vref1, and during a period Tb behind the point A, the selector switch 48 selects the reference voltage Vref2.
FIG. 3 shows the initial state of the reference voltage obtained when the compared input voltage Vin is inputted to the hysteresis detecting circuit 41 in the state in which its voltage level is higher than the voltage levels of the reference voltages Vref1, Vref2. When the aforementioned voltage is inputted to the hysteresis detecting circuit 41, depending upon the relationship between the magnitudes of the voltage in the transient state from the time period in which the hysteresis detecting circuit 41 is actuated to the time period in which the reference voltages Vref1, Vref2 are stabilized and the voltage of the compared input voltage Vin, the output voltage Vout obtained upon actuation of the hysteresis detecting circuit 41 goes to the low level (Low) and the selector switch 48 inputs the low reference voltage Vref1 to the comparator circuit 43 in the initial state. In FIG. 3, during a time period Tc from the actuation of the hysteresis detecting circuit 41 to the time in which the reference voltage is stabilized, the selector switch 48 selects the reference voltage Vref1.
FIG. 4 shows the initial state of a threshold voltage obtained when the compared input voltage Vin is inputted to the hysteresis detecting circuit 41 in the state in which its voltage level lies between the low reference voltage Vref1 and the high reference voltage Vref2. When the aforementioned voltage is inputted to the hysteresis detecting circuit 41, depending upon the relationship between the magnitudes of the voltage in the transient state obtained until the reference voltages Vref1, Vref2 are stabilized after the hysteresis detecting circuit 41 has been actuated and the voltage of the compared input voltage Vin, the output voltage Vout obtained upon actuation of the hysteresis detecting circuit 41 goes to the low level (Low) and the selector switch 48 inputs the low reference voltage Vref1 to the comparator circuit 43 in the initial state.
An illuminance detecting circuit using the above-mentioned hysteresis detecting circuit 41 shown in FIG. 1 will be described next in detail with reference to FIG. 5, in which case a flip-type cell phone is taken as an example.
A flip-type cell phone that is required to consume small power need not energize the KEY display unit in the state in which it is in the closed state (when it is not in use). Thus, when the flip-type cell phone is closed, a power supply voltage is not supplied to the hysteresis detecting circuit that uses the same voltage as that supplied to the KEY display unit. Therefore, in general, at the same time the flip-type cell phone is opened (when it is in use), the power source voltage is supplied to the cell phone to energize the KEY display unit and the hysteresis circuit 41 is actuated. For example, when the intensity of illumination of 100 lux and the intensity of illumination of 150 lux are detected by the hysteresis detecting circuit 41, the low reference voltage Vref1 is set to correspond to 100 lux and the high reference voltage Vref2 is set to correspond to 150 lux. Then, when the output signal from the hysteresis detecting circuit 41 is held at the high level (High), the KEY display unit is energized. When the output signal from the hysteresis detecting circuit 41 is held at the low level (Low), the KEY display unit is deenergized, and hence the KEY display unit can be controlled relative to the ambient illuminance under the circumstances in which the cell phone is in use. It is requested by users that the KEY display unit may be energized at an intensity of illumination lower than 100 lux and that it may be de-energized at an intensity of illumination higher than 150 lux. Also, it is requested that the KEY display unit should be energized only when the cell phone is actuated because the intensity of illumination from 100 to 150 lux changes in response to differences among individuals.
In an illuminance detecting circuit 51 shown in FIG. 5, an illuminance detecting means 56 for detecting ambient illuminance is connected to the input terminal 46 of the comparator circuit 43 in addition to the aforementioned hysteresis detecting circuit 41. The illuminance detecting means 56 is composed of an illuminance sensor 53 formed of a photo-diode, for example, a resistor (R1) 55 connected in series to the illuminance sensor 53 and a capacitance (C1) 54 connected in parallel to the resistor (R1) 55. A connection point between the illuminance sensor 53 and the resistor (R1) 55 is connected to the input terminal 46 of the comparator circuit 43. In FIG. 5, reference numeral 57 denotes a power source of the illuminance sensor 53.
In the illuminance detection and control operations of the illuminance detecting circuit 51 shown in FIG. 5, when the cell phone is opened (when the cell phone is in use) in a place where the ambient illuminance is less than 100 lux, a current corresponding to the ambient illuminance flows from the illuminance sensor 53 to the capacitance (C1) 54 and the resistor (R1) 55, and the voltage Vin corresponding to the intensity of illumination is inputted from the input terminal 46 to the comparator circuit 43. The initial state of the input voltage Vin becomes the high reference voltage Vref2 corresponding to 150 lux based upon the relationship between the magnitudes of the input voltage Vin and the reference voltages Vref1, Vref2 in the transient state. Thus, the illuminance detecting circuit 51 can judge that the ambient illuminance is lower than the intensity of illumination to be detected and can energize the KEY display unit in the way the user wants.
When the cell phone is opened (when the cell phone is in use) in a place where the ambient illuminance is higher than 150 lux, a current corresponding to the ambient illuminance flows from the illuminance sensor 53 to the capacitance (C1) 54 and the resistor (R1) 55, and the voltage Vin corresponding to the intensity of illumination is inputted from the input terminal 46 to the comparator circuit 43. The initial state of the reference voltage becomes the low reference voltage Vref1 corresponding to 100 lux based upon the relationship between the magnitudes of the input voltage Vin and the reference voltages Vref1, Vref2 in the transient state. Thus, the illuminance detecting circuit 51 can judge that the ambient illuminance is higher than the intensity of illumination detected and it can deenergize the KEY display unit in the way the user wants.
Next, when the cell phone is opened (when the cell phone is in use) in a place where the ambient illuminance lies in a range of from 100 to 150 lux, a current corresponding to the ambient illuminance flows from the illuminance sensor 53 to the capacitance (C1) 54 and the resistor (R1) 55, and the voltage corresponding to the intensity of illumination is inputted from the input terminal 46 to the comparator circuit 43. However, since the reference voltages Vref1, Vref2 are slow to rise upon actuation, based upon the relationship between the magnitudes of the input voltage Vin and the reference voltages Vref1, Vref2 in the transient state, the initial state of the reference voltage becomes the low reference voltage Vref1 corresponding to 100 lux. Consequently, the illuminance detecting circuit 51 can judge that the ambient illuminance is higher than the intensity of illumination to be detected. Thus, as shown in FIG. 6, a de-energizing signal P1 containing the initial state is outputted from the output terminal 47 to deenergize the KEY display unit so that the KEY display unit cannot be energized in the way the user wants.
Cited patent reference 1 has described an example of a mobile-device terminal apparatus including an illumination control mechanism.
[Cited patent reference 1]: Official gazette of Japanese laid-open patent application No: 2002-111864
The KEY display unit can be energized in the way the user wants under control of a central processing unit (CPU) as shown in FIG. 7. In FIG. 7, elements and parts identical to those of FIG. 5 are denoted by the identical reference numerals.
As shown in FIG. 7, an illuminance detecting circuit 61 includes the aforementioned hysteresis detecting circuit 41 and the aforementioned illuminance detecting means 56 in which the switch 48 for selecting the reference voltages is controlled by a central processing unit (CPU) 62 incorporated within the cell phone. More specifically, the CPU 62 incorporated within the cell phone can determine whether the cell phone is actuated or whether it is in use. When the cell phone is actuated, the reference voltage set by the switch 48 is constantly set to the high reference voltage Vref2 corresponding to 150 lux under control of the CPU 62. Next, it is determined by the CPU 62 that the output voltage Vout of the comparator circuit 43 is at the high level (High) (that is, when the input voltage Vin is lower than the reference voltages Vref1, Vref2) after a constant time in which the reference voltages Vref1, Vref2 are stabilized since they were actuated, as shown in FIG. 8 (when the input voltage Vin lies between the reference voltages Vref1 and Vref2), and the CPU 62 sets the input voltage to the high reference voltage Vref2 corresponding to 150 lux in the same way as the cell phone is actuated. If it is determined by the CPU 62 that the output voltage Vout of the comparator circuit 43 is held at the low level (Low), as shown in FIG. 10 (when the input voltage Vin is higher than the reference voltages Vref1, Vref2), the CPU 62 sets the switch 48 to the low reference voltage Vref1 corresponding to 100 lux. Also, before the aforementioned constant time t1 passes (that is, the period Td), the CPU 62 outputs the high level (High) voltage as the control signal for energizing or deenergizing the KEY input unit of the cell phone. After the constant time t1 passed (that is, the period Te), the CPU 62 outputs the outputted results of the comparator circuit 62 as the control signal for energizing or de-energizing the KEY display unit of the cell phone. After this, if it is determined by the CPU 62 that the cell phone is in use and then at the same time the output from the comparator circuit 43 is switched, the switch 48 is switched based on an output signal Hys from the CPU 62 to thereby switch the reference voltages Vref1, Vref2 inputted to the comparator circuit 43. When the control signal outputted from the CPU 62 is held at the low level (Low), the KEY display unit is deenergized. When, on the other hand, the above control signal is held at the high level (High), the KEY display unit is energized, whereby the KEY display unit is energized in the way the user wants even in a place where the ambient luminance obtained upon actuation falls within a range of from 100 to 150 lux.
As described above, in the hysteresis detecting circuit according to the prior art, when the compared input voltage is lower than the two reference voltages or when the compared input voltage lies between the two reference voltages, in the initial state from the actuation time to the time until the reference voltage is stabilized, a problem arises, in which the compared input voltage becomes higher than the reference voltage. In order to solve this problem, there is required a terminal for controlling a large-scaled system, such as a CPU, and a reference voltage from the outside. When the CPU or the like is used, it is natural that the circuit arrangement becomes more complex and that its power consumption increases.
In the hysteresis detecting circuit applied to electronic devices such as a mobile device, it can be assumed that the high level and the low level of the output from the comparator circuit will be inverted.